German utility model 88 06 205 describes a slip clutch comprising a first clutch element rotatable about a clutch axis and having a planar surface centered on the axis and formed with at least one axially backwardly open recess and a second clutch element also rotatable about the axis and formed with at least one axially forwardly open window alignable with the recess and having a pair of sides spaced angularly apart by a predetermined distance A coupling ball in the window and of approximately the same diameter as the window width is axially forwardly engageable in the recess A pair of pusher balls in the window axially backward of the coupling body bear axially forward against the coupling body under axial pressure from a spring unit to press the coupling ball into the recess and rotationally couple the clutch elements. When torque between the two elements exceeds a predetermined level, the coupling ball is cammed out of the recess and the two elements can rotate relative to each other.
In this system the position of the coupling ball is defined by the position of the window engaging it and by the position of the pusher balls. This results in excessive friction and wear in the clutch. The transmission of torque between the coupling ball and the walls of the window engaging it creates considerable wear. When the windows wear somewhat, the coupling balls can slip under the pusher balls, at a lower torque level allowing the two clutch elements to rotate relative to each other in the sense of a backlash. It is further known to angle the sides of the windows, but this expedient requires a much stiffer spring unit to be employed, increasing the size and cost of the clutch.